CHARACTERISTICS OF SUPPRESSION OF WHEAT SPINDLE STREAK MOSAIC BY NITROGEN FERTILIZERS

In a field in which 95–100% of the wheat (Triticum aestivum L.) plants developed wheat spindle streak mosaic (WSSM) caused by a soil-borne virus, the incidence of disease was suppressed by preseeding applications of urea, NH 4 NO 3 , or poultry manure, each supplying 500–1,800 kg N/ha, but not by cattle manure supplying up to 1,000 kg N/ha. In growth-room tests, the addition of 5 g NH 4 NO 3 /kg to soil harboring the virus reduced or prevented the infection of wheat plants from the soil, but did not suppress the development of WSSM on plants that were inoculated manually. The disease-reducing effect of the NH 4 NO 3 diminished in soil kept moist at 20–25 C for 30 days before seeding wheat. Since the development of symptoms was not suppressed by NH 4 NO 3 on manually inoculated plants, the effect appeared to result from interference with infection from the soil rather than from suppression of symptom development. The disease-suppressing effects of NH 4 NO 3 and sucrose were reduced when the two treatments were combined in certain proportions.